Provide definitions of all columns in input and output data tables

[hillalex]

The model accepts a table of data with some required columns. These should be documented in full.

It also outputs data tables from the following 3 functions:

• simulate_population_trajectories
• population_stationary_points
• simulate_individual_trajectories

Each of these output a number of columns with non-human readable names. These should be fully documented. Note that for some functions, the columns outputted may be different depending on whether a summarise argument is provided.

All this documentation should live in the data vignette.